An asphyxiant gas is a nontoxic or minimally toxic gas which reduces or displaces the normal oxygen concentration in breathing air. Breathing of oxygen-depleted air can lead to death by asphyxiation (suffocation). Because asphyxiant gases are relatively inert and odorless, their presence in high concentration may not be noticed, except in the case of carbon dioxide (hypercapnia). Toxic gases, by contrast, cause death by other mechanisms, such as competing with oxygen on the cellular level (e.g. carbon monoxide) or directly damaging the respiratory system (e.g. phosgene). Far smaller quantities of these are deadly. Notable examples of asphyxiant gases are methane, nitrogen, argon, helium, butane and propane. Along with trace gases such as carbon dioxide and ozone, these compose 79% of Earth's atmosphere.